Since today's, June 10th, maintenance we have a new feature players may have problems with to understand as it also took me a while to figure it out. :$
I'd like to help you guys with this! ^o^)/

To start off:What are Presets?
Presets are beforehand made text you can save, and directly let it show in chat when selecting the right 'shortcut' for it. There are 15 slots for Presets in total.
If you have ever played the WvW Section, you'd be familiar with this as there is also an option to Presets there; like 'Don't act alone!' / 'Stay as a group!' / 'Everyone defend the core!' / 'Attack enemy's Obeliks!' etc.

Now how to set Presets and how to use them?
I'll explain this with some text and screens!! ( ^O^)7

When you're logged in, the first thing you have to do it to find the config! Menu > Options > Config.

Now we have opened the Config Menu, we have to switch to Chat! When you've chosen chat you need to scroll the right side a bit to the bottom.

Here you see "Default Chat Style", "Input" and "Edit Presets".
Tap "Edit Presets" and you are able to edit the already existing Texts for the Presets. Simply tap on one of the texts there and you can edit the text.
(Note: in the screens I have already edited 1-9 Presets. Presets 10-15 are still the default texts xD).
(Default text of the first was: 1 Hello | which I changed to: 1 Hi~)

When you're done with the edits, you have to close the screen and then save/confirm your config.

So, now you should be able to use the Presets! (If this isn't the case, try to change the "Input" to "Show Presets" [option above "Edit Presets"]).
Try to 'Hide' your chat by tapping the grey/yellow icon beside "Talk".
(No need to worry!! At the 3rd tap the chat is hidden!! ^^)

Here you see three options, whereas each Preset contains 5 Preset Options, if you tap on it!